The present invention relates to a method in connection with converter bridges connected in parallel, whereby the converter bridges are directly connected in parallel and the converter bridges are controlled independently on the basis of control variables, the method comprising a step of controlling the converter bridges to produce an output switching vector on the basis of the control variables.
Converter bridges implemented with IGB transistors are used in modern electric drives both as motor converters and network converters. Network converters are typically used when a drive must have four-quadrant operation. The power range of converters used is wide, varying from a few dozens of kilowatts to several megawatts. Although current-carrying capacities and maximum voltages of IGB transistors have risen continuously, power stages must still be connected in parallel when high power is used in order to secure the current-carrying capacity of transistors. It is also possible to have a redundant parallel connection, which aims at improving the usability of an electric drive and securing the continuity of operation also when a converter bridge or a few converter bridges fail.
Previously, converter bridges controlled on the basis of a principle of direct torque control (DTC) could be connected in parallel in both motor and network converter applications by utilizing synchronized transistor control shown in FIG. 1, i.e. by copying switching instructions given by one control card to all parallel power stages. In such a case, electrical properties of semi-conductor power switches must be as similar as possible so that the currents would be distributed evenly between the parallel transistors. It is often difficult and cumbersome to find components that are sufficiently alike. In addition, if a control card 1 fails, the entire system does not function any more. Therefore, it is also impossible to implement a redundant parallel connection by using the principle of FIG. 1. Another disadvantage of the solution is that a fully modular design of the system is not possible.
In case of network converters, another alternative has been that a galvanic isolation, i.e. a supply transformer with several three-phase secondary windings, is used on the side of a supplying alternating-current network as in FIG. 2. Such a supply transformer is, however, very expensive and a large separate component.
In motor applications, a method corresponding to FIG. 2 is to use a motor with three-phase stator windings the number of which corresponds to parallel motor converter units. The galvanic isolation of the solution thus requires a specially-built, expensive motor. A redundant parallel connection by using the solution of FIG. 2 is not economically reasonable, because each unit, depending on the application, requires its own secondary windings in a supply transformer or stator windings in a motor.
The publication Ogasawara S., Takagaki J., Akagi H., Nabae A., “A Novel Control Scheme of a Parallel Current Controlled PWM Inverter”, IEEE Transactions on Industry Applications, Vol. 28, No. 5, September/October, 1992, pages 1023-1030, discloses a method for parallel connection of motor converters, based on the use of current balancing chokes, the method being not applied in connection with direct torque control, however.
The publication Ye Z., Boroyevich D., Choi J-Y., Lee F. C., “Control of Circulating Current in Parallel Three-Phase Boost Rectifiers”, Record of APEC 2000 Conference, Vol. 1, 2000, pages 506-512, describes a method for controlling circulating current in converter bridges connected in parallel and using a pulse-width modulation (PWM) modulator, the method being not applicable in connection with DTC control, however.